For the purpose of the USCG License exams, all lines are measured by circumference. To convert use the formula's below.
The basic breaking strength factor for manila line is found by multiplying the square of the circumference of the line by 900 lbs.
(900 lbs. X circumference2 = breaking strength)
Circumference = p PI (3.14) X diameter
Example, if you had a piece of ½" manila line and wanted to find the breaking strength, you would first calculate the circumference. (.5 X 3.14 = 1.57) Then using the formula above:
1.572 X 900 = 2,218 pounds of breaking strength
To calculate the breaking strength of synthetic lines you need to add one more factor. As mentioned above, a comparison factor has been developed to compare the breaking strength of synthetics over manila. Since synthetics are stronger than manila an additional multiplication step is added to the formula above.
(comparison factor X 900 lbs. X circumference2 = breaking strength)
Comparison chart for synthetic lines.
Line Material | Comparison Factor (greater than manila) |
| Nylon | 2.5 |
| Dacron | 2.0 |
| Polypropylene | 1.4 |
Using the example above, the breaking strength of ½" nylon line. First convert the diameter to circumference, then write the formula including the extra comparison factor step.
2.5 X 1.572 X 900 = 5, 546 pounds of breaking strength
Knots and splices will reduce the breaking strength of a line by as much as 50 to 60 percent. The weakest point in the line is the knot or slice, but a splice is stronger than a knot.
